1. Field of the Invention
The present invention relates generally to the field of forming metal foils and, more particularly, to corrugating metal foil strips for use in fabricating a metal honeycomb core material for lightweight honeycomb sandwich panels.
2. Background Information
Honeycomb sandwich panels are in widespread use in, for example, aerostructures. Producing the core material for honeycomb sandwich panels having a honeycomb core of a metal, such as aluminum or titanium, requires corrugating thin foil strip material for assembly of the honeycomb core. Two basic types of machines have previously been used for corrugating metal foil strips.
In one type of machine, foil strip material passes between a pair of rotating, meshing forms gears and in so doing is corrugated as the teeth of one gear progressively push segments of the foil into the cavities between the teeth of the other gear. One significant problem with gear/gear machines is maintaining close tolerances, especially the pitch of the nodes where adjacent corrugated strips are joined to each other to form the honeycomb material. The corrugations are subject to variations in their shapes and dimensions due to some elastic rebound of the corrugations after they are formed and variations in the pressure acting on the strip material as it is progressively pressed into a cavity between adjacent teeth of one form gear by a tooth of the other form gear. Because both form gears are rotating at the time of forming a corrugation, the tooth of one form gear rocks about its tip relative to the cavity of the other form gear that receives it. The rocking of the teeth also imposes limitation on the shapes and depths of the corrugations. An advantage of gear/gear machines is that the gears feed the incoming foil stock by drawing it into the meshing portion and pushing the corrugated strip material out as the gears rotate.
Strips of metal foil for making honeycomb core material have also previously been corrugated by sequentially pressing segments of the strip into the cavity of a form die by a reciprocating punch. Commonly, the punch has several teeth and the die several cavities, the teeth and cavities being successively larger and deeper in the feed direction so that each corrugation is formed progressively in stages. Generally, a well designed xe2x80x9cpunch/diexe2x80x9d corrugating machine is capable of greater precision than a gear/gear machine because the working of the strip material is unidirectional and the strip is stationary during working. A punch/die machine is, however, more complicated than a gear/gear machine, inasmuch as the strip has to be fed by a hitch feed mechanism, which pushes the stock one pitch distance into the punch/die forming part of the machine and dwells while the punch moves in the working stroke. Punch/die machines also rely on rebounding of the strip from the die to clear the nodes from the die so that the strip can be indexed to bring a new segment between the punch and die. Operating speeds are limited due to the relatively large masses of the tools and the need to index the foil strip after each operating cycle of the punch. For any given machine, there is no way of controlling the punch force, which would be desirable to accommodate foils of different materials and thicknesses. Generally, the machines are designed to handle the least ductile and thickest materials being corrugated. More ductile and thinner materials are, therefore, subjected to excessive forces. The forming tools wear rapidly and are subject to fatigue fracture. Large amounts of lubricant are required on the forming surfaces to minimize friction between the foil stock and the forming tools.
In view of the disadvantages of gear/gear and punch/die corrugating machines, there is a need for a machine for corrugating a metal foil strip with greater precision than can be readily attainable with gear/gear machines and at greater speeds and with less complicated machine elements than are ordinarily found in previously known punch/die machines. There is also a need for a corrugating machine in which the operating parameters can be readily controlled, preferably on the fly. It is also desired to have a corrugating machine that can form corrugations of more complex shapes and/or with nodes of greater depth than those that can be formed with previously known machines.
The foregoing needs are fulfilled, in accordance with the present invention, by a machine for corrugating a metal foil strip that includes a form gear having equally spaced-apart identical teeth defining a multiplicity of identical cavities along the circumference of the form gear, a rotary drive rotating the form gear intermittently to index successive cavities to a forming station along the perimeter of the form gear where a corrugation is at least partly formed in a cavity then at the forming station while the rotary drive dwells, and a punch unit located at the forming station for forming corrugations in the strip one by one. The punch unit has a form tooth receivable seriatim in the cavities of the form gear and adapted thereby to at least partly form corrugations in the foil strip, and a reciprocating linear actuator driving the form tooth radially of the form gear in a succession of forming cycles. Each of the forming cycles includes a forming stroke in which the form tooth moves into a cavity then at the forming station, a return stroke in which the form tooth moves out of the cavity then at the forming station, and a dwell period in which the form tooth dwells while the form gear indexes.
One very significant advantage of a machine according to the present invention is that the form gear is stationary as each corrugation is formed by a unidirectional forming stroke of the form tooth. Accordingly, the corrugations may be formed with greater precision than is generally attainable with a gear/gear machine. The form tooth and the cavities of the form gear may be closely complementary in shape, because there is no rocking motion of each tooth of one form gear relative to the cavity of the other form gear. A machine according to the present invention is of a construction that is simpler than that of a punch/die machine, inasmuch as the form gear transports the outgoing corrugated strip away from the forming station as it successively indexes new segments of foil to the forming station. Accordingly, a strip transport mechanism separate from the punch/die unit or mechanisms for moving the punches in the direction of the path of the strip to index corrugated sections of the strip away from the forming station and transport another section from the supply to the forming station are not required. The form tooth may be kept small and light in weight and can thus be relatively rapidly accelerated and decelerated during each forming cycle, thereby permitting a higher operating speed than is conventionally available in known gang-type punch/die machines.
In preferred embodiments, the punch unit further includes a hold finger that is engageable with an outgoing node of a corrugation of the strip and holds the outgoing node against the tip of the tooth of the form gear on the outgoing side of the cavity then at the forming station during a substantial portion of the duration of each forming cycle of the punch. The hold finger feature prevents the outgoing corrugation from being pulled back partly into the cavity then at the forming station and ensures that the next corrugation is formed solely from a segment from the incoming foil supply. The resulting precise displacement into the cavity of a segment drawn solely from the incoming strip and the elimination of any pull-back of part of the outgoing node contribute to more consistent formation of the corrugations.
A machine according to the invention, preferably, includes an electronically-controlled linear servomotor driving the punch and an electronically-controlled rotary servomotor driving the form gear. Servo drives permit on-the-fly control of the operating speed, the force applied to the strip by the form tooth, and the timing of each portion of the operating cycle. The machine can be controlled by relatively simple computer programs. It is desirable to place the drives in a master-slave relationship.
Linear servomotors have driveshafts that are in lateral clearance from the driving coils. Accordingly, the drive shaft does not provide precise axial movement of the tool that it drives. In order to move the form tooth precisely into the cavity of the form gear, the punch unit includes a fixed tool guide having a guideway receiving a portion of the form tooth for guided axial movement. A tool support is affixed to the output shaft of the linear actuator, and the form tooth is carried by the tool support for universal swiveling motion relative to the tool support. The swivel connection allows the form tooth to be guided by the tool guide into the cavity without binding, even though the drive shaft inherently displaces slightly from axial alignment with the nominal center axis of the linear servomotor.
It is desirable to include a foil exit guide shoe extending along a segment of the form gear immediately downstream of the forming station and having a guide surface in close clearance with outer nodes of an outgoing corrugated portion of the foil strip so as to retain the outgoing corrugated portion of the foil strip in a multiplicity of cavities of the form gear. Such an exit guide holds several nodes of the corrugated foil strip immediately downstream from the forming station in place on the form gear for reliable transport of the corrugated part of the strip from the forming station and of the supply strip to the forming station.
The present invention also includes embodiments in which each corrugation is formed in two (or more) stages. In a two-stage machine, for example, the form tooth of a first-stage punch unit is shaped to only partially form the corrugations, and the machine has a second-stage punch unit located at a second forming station along the circumference of the form gear that is spaced apart from the first-stage forming station. The second-stage punch unit has a form tooth that is receivable seriatim in the cavities of the form gear and shaped to fully form the corrugations in the foil strip and a second reciprocating linear actuator driving the second form tooth radially of the form gear intermittently in a succession of forming cycles, each of which includes a forming stroke in which the second form tooth moves into a cavity then at the second forming station and a return stroke in which the second form tooth moves out of the cavity then at the second forming station, each forming cycle of the second form tooth being concurrent with the forming cycle of the punch unit.
The second-stage punch unit, preferably, includes a pair of hold fingers. One hold finger is arranged to engage an incoming node of a partly formed corrugation of the strip against the tip of the tooth of the form gear on the incoming side of the cavity then at the second forming station during a substantial portion of the duration of each forming cycle of the second-stage form tooth. The other hold finger is arranged to engage an outgoing node of a corrugation of the strip against the tip of the tooth of the form gear on the outgoing side of the cavity then at the second forming station during a substantial portion of the duration of each forming cycle of the second form tooth. Holding the nodes against the teeth on either side of the cavity then at the forming station ensures that the second-stage forming of each corrugation involves evenly pressing the node being formed without any pull forward of a partially formed, incoming corrugation or pull back of a fully-formed, outgoing corrugation.
The foregoing description has outlined rather broadly some features and advantages of the present invention. The detailed description of embodiments of the invention that follows will enable the present invention to be better understood and the present contribution to the art to be more fully appreciated. Those skilled in the art will recognize that the embodiments may be readily utilized as a basis for modifying or designing other structures for carrying out the purposes of the present invention. All such structures are intended to be included within the spirit and scope of the invention as set forth in the appended claims.